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We report the discovery of a circumplanetary disk (CPD) candidate embedded inthe circumstellar disk of the T Tauri star AS 209 at a radial distance of about200 au (on-sky separation of 1."4 from the star at a position angle of$161^\circ$), isolated via $^{13}$CO $J=2-1$ emission. This is the firstinstance of CPD detection via gaseous emission capable of tracing the overallCPD mass. The CPD is spatially unresolved with a $117\times82$ mas beam andmanifests as a point source in $^{13}$CO, indicating that its diameter is$\lesssim14$ au. The CPD is embedded within an annular gap in the circumstellardisk previously identified using $^{12}$CO and near-infrared scattered lightobservations, and is associated with localized velocity perturbations in$^{12}$CO. The coincidence of these features suggests that they have a commonorigin: an embedded giant planet. We use the $^{13}$CO intensity to constrainthe CPD gas temperature and mass. We find that the CPD temperature is$\gtrsim35$ K, higher than the circumstellar disk temperature at the radiallocation of the CPD, 22 K, suggesting that heating sources localized to the CPDmust be present. The CPD gas mass is $\gtrsim 0.095 M_{\rm Jup} \simeq 30M_{\rm Earth}$ adopting a standard $^{13}$CO abundance. From the non-detectionof millimeter continuum emission at the location of the CPD ($3\sigma$ fluxdensity $\lesssim26.4~\mu$Jy), we infer that the CPD dust mass is $\lesssim0.027 M_{\rm Earth} \simeq 2.2$ lunar masses, indicating a low dust-to-gas massratio of $\lesssim9\times10^{-4}$. We discuss the formation mechanism of theCPD-hosting giant planet on a wide orbit in the framework of gravitationalinstability and pebble accretion.

Molecules with ALMA at Planet-forming Scales (MAPS). A Circumplanetary Disk Candidate in Molecular Line Emission in the AS 209 Disk